At present, conventional edge emitting semiconductor lasers play a significant role in optical communication due to their high operating efficiency and modulation capabilities, however, edge emitting semiconductor lasers have several short comings or problems, thus making them difficult to use in several applications.
Recently, there has been an increased interest in vertical cavity surface emitting lasers (VCSELs). The conventional VCSEL has several advantages, such as emitting light perpendicular to the surface of the die, and the possibility of fabrication of two dimensional arrays. However, while conventional VCSELs have several advantages, they also have several disadvantages with regard to emission in the visible spectrum primarily due to the poor reflectivity of the distributed Bragg reflectors which are contained as a part of the VCSEL structure and degradation of structural materials during the fabrication process. Because of this, manufacturability of VCSELs for the visible spectrum is severely limited.
Short wavelength laser diodes are of great interest for high density optical data storage, medical applications, etc. With the emergence of digital video disk (DVD) technology which utilizes visible wavelength semiconductor lasers for data storage, the market demand for both 635 nm and 650 nm semiconductor lasers is expected to soon catch up with the demand for the now common 780 nm compact disk (CD) lasers.
A typical structure of a now existing vertical cavity surface emitting laser is illustrated in FIG. 1 and labeled prior art. Specifically, illustrated is a simplified cross-sectional view of a portion of a vertical cavity surface emitting laser 10. Vertical cavity surface emitting laser 10 is fabricated on a semiconductor substrate 12, more particularly a gallium arsenide substrate. A first stack of distributed Bragg reflectors 14, comprised of a plurality of alternating layers 16 is positioned on a surface 15 of semiconductor substrate 12. The plurality of alternating layers 16 of first stack of distributed Bragg reflectors arsenide material and a n-doped gallium aluminum arsenide material. There is next fabricated a cladding region 26, on a surface of first stack of distributed Bragg reflectors 14, an active region 20 disposed on cladding region 26, and a cladding region 27 disposed on a surface of active region 20. A second stack of distributed Bragg reflectors 22 is positioned on a surface of cladding region 27. Second stack of distributed Bragg reflectors 22 is formed of a plurality of alternating layers 23, more specifically alternating layers 24 and 25 of a p-doped aluminum arsenide and a p-doped gallium aluminum arsenide. Second stack of distributed Bragg reflectors 22 is followed by a one-half wavelength aluminum gallium arsenide contact layer 28. Contact layer 28 is p-doped to 1E19 cm.sup.-3 or higher. Finally, a very thin gallium arsenide cap layer 30 is positioned on a surface of contact layer 28. Cap layer 30 is very thin, more specifically on the order of 100 .ANG. thick. Cap layer 30 is p-doped to 1E19 cm.sup.-3 or higher.
There exist several drawbacks to this type of vertical cavity surface emitting laser, more particularly VCSEL 10. Of particular concern is one-half wavelength contact layer 28. As stated, contact layer 28 in this particular embodiment is fabricated of aluminum gallium arsenide. More specifically, contact layer 28 is fabricated to include approximately fifty percent aluminum. Aluminum is a wide band gap material and very difficult to dope to a level of 1E19 cm.sup.-3 with zinc or carbon. While doping of contact layer 28 with zinc or carbon can be achieved at growth temperatures around 550.degree. C.-600.degree. C., at this extreme temperature, the quality of contact layer 28 degrades rapidly due to the oxygen incorporation. Another drawback to contact layer 28 is that only a thin layer of gallium arsenide in cap layer 30, more particularly a layer of 100 .ANG. or less, protects the surface of contact layer 28. Cap layer 30 can be easily removed and/or damaged during the process of fabrication of VCSEL 10 which results in a rapid oxidation of VCSEL 10 structure, including contact layer 28, second stack of distributed Bragg reflectors 22 structure, active region 20 and first stack of distributed Bragg reflectors 14 structure.
Thus, there is a need for developing visible light emitting vertical cavity surface emitting laser (VCSEL) for use in high density DVD technologies that includes an easily doped contact layer, thereby maintaining structural integrity of the VCSEL device.
Accordingly, it is highly desirable to provide for a visible light emitting vertical cavity surface emitting laser (VCSEL) for use in high density DVD technologies that includes the fabrication of a VCSEL structure on a semiconductor substrate and includes a one-half wavelength contact layer within the VCSEL structure that is easily p-doped to a level of 1E19 cm.sup.-3 or higher.
It is a purpose of the present invention to provide a new and improved vertical cavity surface emitting laser that utilizes a gallium phosphide contact layer, thereby capable of being easily p-type doped with a material such as magnesium, zinc, or a combination of magnesium and zinc without damage to the underlying VCSEL structure.
It is a further purpose of the present invention to use a gallium phosphide contact layer, as part of a VCSEL structure that further serves as a passivation layer.
It is a still further purpose of the present invention to provide for a new and improved vertical cavity surface emitting laser that is capable of emission in the visible spectrum.